The original M16 was the one without the forward assist. The M16A1 added the forward assist. A myriad of changes arrived with the M16A2, some no necessarily for the better. The most annoying of which was an inconsistent trigger pull due to the burst cam. This is not a problem with any semi-auto only civilian models though. The other annoyance is that they changed the buttstock length.
Every other change (twist rate, barrel thickness, hand guards, sights - front AND rear, SHAPE OF FORWARD ASSIST - yes, there is a change evident even on the pictures someone posted, pistol grip, buttstock latch and type of plastic used for the stock) was, in my opinion, an improvement. The sights are no less sturdy. They have held up fine from Panama to Iraq to Somalia - they will hold up fine at the range.
Adam